Iowa State University Extension

What you need to know about the leading cause
of poisoning deaths in America

Carbon Monoxide


The following text is from the booklet What you need to know about the leading cause of poisoning deaths in America: Carbon monoxide (1996), prepared as a public service by First Alert in cooperation with the Cooperative State Research, Education, and Extension Service (CSREES) of the U.S. Department of Agriculture.

If you have questions about carbon monoxide, contact your local gas utility, a qualified heating contractor, or the Extension Services office listed in the white pages of your phone book.


What is carbon monoxide and who is at risk?

Carbon monoxide (CO) is a colorless, odorless deadly gas. Because you can't see, taste, or smell it, carbon monoxide can kill you before you know it's there.

Everyone is at risk for carbon monoxide poisoning. Experts believe, however, that individuals with greater oxygen requirements such as unborn babies, infants, children, senior citizens, and people with coronary or respiratory problems are at greater risk.

 

Why is carbon monoxide so dangerous?

The great danger of carbon monoxide is its attraction to hemoglobin in the bloodstream. When breathed in, carbon monoxide replaces the oxygen which cells need to function. When CO is present in the air, it rapidly accumulates in the blood, causing symptoms similar to the flu, such as headaches, fatigue, nausea, dizzy spells, confusion, and irritability. As levels increase, vomiting, loss of consciousness, and eventually brain damage or death can result.

 

Where does carbon monoxide come from?

Carbon monoxide is a by-product of combustion, present whenever fuel is burned. It is produced by common home appliances, such as gas or oil furnaces, gas refrigerators, gas clothes dryers, gas ranges, gas water heaters or space heaters, fireplaces, charcoal grills, and wood burning stoves. Fumes from automobiles and gas-powered lawn mowers also contain carbon monoxide and can enter a home through walls or doorways if an engine is left running in an attached garage.

All of these sources can contribute to a CO problem in the home. If a home is vented properly and is free from appliance malfunctions, air pressure fluctuations or airway blockages, carbon monoxide will most likely be safely vented to the outside. But in today's "energy efficient" homes this is frequently not the case. Tightly constructed/sealed homes can trap CO-polluted air in a home year-round. Furnace heat exchangers can crack, vents can become blocked, inadequate air supply for combustion appliances can cause conditions known as backdrafting or reverse stacking, which force contaminated air back into the home. Exhaust fans on range hoods, clothes dryers and bathroom fans can also pull combustion products into the home.

 

How can I protect myself and my family from carbon monoxide poisoning?

The Consumer Product Safety Commission (CPSC) recommends installing at least one carbon monoxide detector per household, near the sleeping area. Additional detectors on every level of a home and in every bedroom provide extra protection. Choose an Underwriters Laboratories Inc. (UL) listed detector that sounds an audible alarm. You can choose a model that is wired to your home's electrical system, a model which plugs into a standard electrical outlet, or a battery-operated model. Battery-operated carbon monoxide detectors continue to protect even in the event of a power outage. Hardwired AC models, although more costly and difficult to install, reduce the expense of battery replacement but do not offer protection during power outages. Hardwired AC models with battery back-up offer double protection.

Gas appliances should be serviced yearly by a qualified service technician. Stove burners should be cleaned and adjusted to minimize the amount of carbon monoxide produced. Before making changes to a house that might affect the ventilation of fuel-burning appliances, contact your heating contractor. When replacing heating appliances, purchase appliances designed to reduce dangers from carbon monoxide, such as sealed combustion gas furnaces, direct vent gas fireplaces, or induced draft gas water heaters. Electric-powered heating appliances do not produce carbon monoxide.

If your carbon monoxide detector sounds, first make sure it is your CO detector and not your smoke detector. The latest generation of carbon monoxide detectors listed with UL will be marked "carbon monoxide detector" in a contrasting color on the cover. Some detectors feature a warning alarm which will sound before the full (continuous) alarm. If your detector is in warning alarm, carbon monoxide is beginning to accumulate. It is important to locate and eliminate the source of CO before the condition worsens. A trained HVAC contractor or appliance service technician can inspect your home to determine the cause of carbon monoxide build-up.

If your detector is in full alarm and any member of the household is experiencing symptoms of poisoning, call your local fire department immediately! Remember that infants and children may be affected more quickly by carbon monoxide. Be sure to see if they are exhibiting symptoms. If no one is feeling symptoms, ventilate the home with fresh air, turn off all potential sources of carbon monoxide and have a qualified technician inspect your combustion appliances.

In addition to installing carbon monoxide detectors, consumers should regularly inspect and service potential problem sources of carbon monoxide.

 

Checklist -- Where to look for problem sources of carbon monoxide:

Furnaces are frequently the source of leaks and should be carefully inspected. Have a professional check the following:

  • Measure the concentration of CO in the flue gases.
  • Check furnace connections to flue pipes and venting systems to outside of the home for signs of corrosion, rust, gaps, or holes.
  • Check furnace filters and filtering systems for dirt or blockages.
  • Check forced air fans for proper installation and correct air flow of flue gases. Improper furnace blower installation can result in carbon monoxide build-up because toxic gas is blown into rather than out of the house.
  • Check the combustion chamber and internal heat exchanger for cracks, metal fatigue or corrosion--be sure they are clean and free of debris.
  • Check burners and ignition system. A flame that is mostly yellow in color in natural gas-fired furnaces is often a sign fuel is not burning completely and higher levels of carbon monoxide are being released. Oil furnaces with similar problems can give off an "oily" odor. Remember, you can't smell carbon monoxide.


Check all venting systems to the outside, including flues and chimneys for proper design and installation, cracks, corrosion, holes, debris, or blockages. Animals and birds can build nests in chimneys, preventing gases from escaping.

Check all other appliances that use flammable fuels such as natural gas, oil, wood, or kerosene.

  • Appliances include water heaters, clothes dryers, kitchen ranges, ovens or cooktops, wood burning stoves, gas refrigerators.
  • Pilot lights can be a source of carbon monoxide because the by-products of combustion are released inside the home rather than vented to the outside. Gas ovens and ranges should be monitored closely.

Be sure space heaters are vented properly. Unvented space heaters that use a flammable fuel such as kerosene can release carbon monoxide into the home.

Barbecue grills should never be operated indoors, nor should stove tops or ovens that operate on flammable fuels ever be used to heat a residence.

Check fireplaces for closed, blocked or bent flues, soot, and debris.

Check the clothes dryer vent opening outside the house for lint.

If initial testing does not confirm the presence of carbon monoxide, there may be several reasons.

Testing equipment used to measure the presence of carbon monoxide in the air must be calibrated to sense low levels of gas concentration.

  • Some detection devices only measure concentrations of 1,000 parts per million and higher, significantly above safe levels. Testing equipment should be capable of sensing levels as low as one part per million. For example, Underwriters Laboratories' standard for residential carbon monoxide detectors requires detectors to alarm before 90 minutes of exposure to 100 parts per million of carbon monoxide.
  • If initial readings don't reveal sufficient concentrations of carbon monoxide to set off the alarm, digital measurement testing equipment that produces a printed 24-hour record can be used to help identify the source.

If doors or windows are left open or appliances are turned off and outside air enters the home, carbon monoxide can dissipate. This creates a lower reading than the level that triggered the alarm.

  • To help assure proper measurement, carbon monoxide readings should be conducted as soon as possible after an alarm incident.

If appliances, flues and chimneys are confirmed to be in good working order, the source of carbon monoxide leaks may be from backdrafting.

This condition exists primarily in tightly sealed/constructed homes. Flue gases normally vent to the outside through flues and chimneys. Air pressure inside a tightly sealed home may become lower than outside, causing outside air to flow into the house through vents and chimneys.

Inadequate air supply in a room where two or more combustion-driven appliances share the same air source, such as a water heater and furnace in a utility closet, can create a more complicated form of backdrafting called reverse stacking.

  • This occurs when one appliance, such as the furnace, turns on and is unable to get adequate fresh air. When the furnace operates, it then draws contaminated air from the water heater exhaust and spreads polluted air throughout the house.

Note: Because carbon monoxide accumulates in some detectors over time, as it does in the bloodstream, the source of CO may be appliances that were running before the alarm sounded.

A sticking thermostat can keep the furnace running continually, depleting the oxygen supply inside the house. This can lead to backdrafting.

In multiple family dwellings where living spaces share walls and pipes, carbon monoxide from one unit may enter a neighboring space through floor boards, cracks or underneath doors.

If a home has an attached garage, carbon monoxide produced by car exhaust can leak into the house. This is especially a problem for home mechanics who may run the car engine frequently for periods of time--even if the garage door is left open.


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Page last updated Aug. 9, 1999